Conductive Polymer Dispersions for Solid Electrolytic Capacitors

ABSTRACT

A capacitor with an anode and a dielectric over the anode. A first conductive polymer layer is over the dielectric wherein the first conductive polymer layer comprises a polyanion and a first binder. A second conductive polymer layer is over the first conductive polymer layer wherein the second conductive polymer layer comprises a polyanion and a second binder and wherein the first binder is more hydrophilic than the second binder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to pending U.S. Provisional ApplicationNo. 61/489,571 filed May 24, 2011 which is incorporated herein byreference.

BACKGROUND

The present invention is related to an improved method of forming asolid electrolyte capacitor and an improved capacitor formed thereby.More specifically, the present invention is related to an improvedcathode where the cathode comprises highly conductive polymer dispersioncoatings with enhanced reliability. The invention also discloses methodsfor manufacturing the intrinsically conductive polymer dispersions. Thepresent invention is related to an improved method of forming a solidelectrolyte capacitor and an improved capacitor formed thereby. Morespecifically, the present invention is related to a capacitor withimproved leakage stability.

The construction and manufacture of solid electrolyte capacitors is welldocumented. In the construction of a solid electrolytic capacitor avalve metal serves as the anode. The anode body can be either a porouspellet, formed by pressing and sintering a high purity powder, or a foilwhich is etched to provide an increased anode surface area. An oxide ofthe valve metal is electrolytically formed to cover all surfaces of theanode and serves as the dielectric of the capacitor. The solid cathodeelectrolyte is typically chosen from a very limited class of materials,to include manganese dioxide or electrically conductive organicmaterials such as polyaniline, polypyrrole, polyethylenedioxythiopheneand their derivatives. Solid electrolytic capacitors with intrinsicallyconductive polymers as the cathode material have been widely used in theelectronics industry due to their advantageous low equivalent seriesresistance (ESR) and “non-burning/non-ignition” failure mode. In thecase of conductive polymer cathodes the conductive polymer is typicallyapplied by either chemical oxidation polymerization, electrochemicaloxidation polymerization or spray techniques with other less desirabletechniques being reported.

The backbone of a conductive polymer consists of a conjugated bondingstructure. The polymer can exist in two general states, an undoped,non-conductive state, and a doped, conductive state. In the doped state,the polymer is conductive but of poor processibility due to a highdegree of conjugation along the polymer chain, while in its undopedform, the same polymer loses its conductivity but can be processed moreeasily because it is more soluble. When doped, the polymer incorporatesanionic moieties as constituents on its positively charged backbone. Inorder to achieve high conductivity, the conductive polymers used in thecapacitor must be in doped form after the completion of the process,although during the process, the polymer can be undoped/doped to achievecertain process advantages.

Various types of conductive polymers including polypyrrole, polyaniline,and polyethyldioxythiophene are applied to the Ta capacitors. The majordrawback of conductive polymer capacitors, regardless of the types ofconductive polymers employed, is their relatively low working voltagecompared to their MnO₂ counterparts. The polymer capacitors havereliability issues, to varying degrees, when the voltage rating exceeds25V. This is believed to be caused by the relatively poordielectric-polymer interface, which has poor “self-healing” capability.The ability to withstand high voltage can be best characterized by thebreakdown voltage (BDV) of the capacitors. Higher BDV corresponds withbetter reliability. For reasons which were previously unknown the breakdown voltage of capacitors comprising conductive polymers has beenlimited to about 55V thereby leading to a capacitor which can only berated for use at about 25V. This limitation has thwarted efforts to useconductive polymers more extensively.

U.S. Pat. No. 7,563,290, which is incorporated herein by reference,describes the slurry/dispersion process. The resulted capacitors showexcellent high voltage performances, reduced DC leakage (DCL) andimproved long term reliability.

It is highly desirable that the capacitor devices are of highreliability and that they can withstand stressful environments.Therefore, the integrity of the anodes and the robustness of conductivepolymer cathode are essential for high quality capacitor products.However, it is a challenge to form a conductive polymer coating on theanodes that is defect-free, and which is capable of withstanding thermalmechanical stress during anode resin encapsulation and surface-mounting.The improper application of polymer slurry often leads to the formationof cracks and delaminating of the polymer coating thus formed.

In a manufacturing process to produce conductive polymer based valvemetal capacitors the valve metal powder, such as tantalum, ismechanically pressed into. anodes that are subsequently sintered to formporous bodies. The anodes are anodized to a pre-determined voltage in aliquid electrolyte to form a dielectric layer onto which a cathode layerof conductive polymer is subsequently formed via an in situpolymerization process comprising multi-cycle oxidizer/monomer coatingsand polymerization reactions. The anodes are then coated with graphiteand Ag followed by assembling and molding into a finished device.

Today, almost all electronic components are mounted to the surface ofcircuit boards by means of infra-red (IR) or convection heating of boththe board and the components to temperatures sufficient to reflow thesolder paste applied between copper pads on the circuit board and thesolderable terminations of the surface mount technology (SMT)components. A consequence of surface-mount technology is that each SMTcomponent on the circuit board is exposed to soldering temperatures thatcommonly dwell above 180° C. for close to a minute, typically exceeding230° C., and often peaking above 250° C. If the materials used in theconstruction of capacitors are vulnerable to such high temperatures, itis not unusual to see significant shifts in ESR and leakage which leadto negative shifts in circuit performance.

The state of the art for inherently conductive polymer (ICP) dispersionshas a number of issues when solid electrolytic capacitors withconductive polymer dispersions are exposed to SMT conditions.

The presence of moisture in the materials used in capacitors can causepoor package integrity due to SMT reflow conditions. Conductive polymerdispersions have relatively high moisture sorption in comparison withinsitu polymerized conductive polymer. Presence of hydrophilicpolyanions, specifically polystyrene sulfonic acid, is one of the reasonfor the high moisture sorption. Insitu polymerized parts use monomericdopant in comparison to polymeric dopants, polyanions, used indispersions. When heated to a temperature higher than the boiling pointof water, which occurs during mounting of the capacitor on the mountingsubstrate by solder reflow, moisture contained in the capacitor elementof the capacitor is vaporized which increases the internal pressure ofthe mold resin. Since the capacitor is rapidly heated to the solderreflow temperature, which is substantially higher than the boiling pointof water, the internal pressure of the capacitor is increasedsubstantially and rapidly. In such cases, since the capacitor element iscompletely encapsulated by the humidity resistant mold resin of such asepoxy resin, vapor thus generated in the capacitor cannot escape throughthe mold resin, so that all high pressure due to water vapor is exertedon the mold resin. As a result, portions of the mold resin are crackedand water vapor in the mold resin release through the cracks. This isparticularly a problem in thin portions of the resin such as nearconnection portions and on the lower surface side.

U.S. Pat. No. 6,229,688 discloses a method to reduce case integrity/casecracking by providing a water release mechanism. The solid electrolyticcapacitor features a water vapor passage formed in the mold resin. Thewater vapor discharge passage is formed of a material having higherwater vapor permeability than that of the mold resin and functions tocommunicate an interior of the mold resin to atmosphere.

A number of approaches are reported to improve moisture resistance ofthe materials in the capacitor. One approach to improving moistureresistance is provided in U.S. Publ. Pat. Appl. No. 20100254072 whereinconductive polymer dispersions and solid electrolytic capacitors aretaught to have a low ESR and an excellent moisture resistance due toincorporation of a sulfonic acid ester compound in the conductingpolymer.

Another approach is provided in U.S. Publ. Pat. Appl. No. 2011/0019340where the electrically conductive polymer suspension comprises dopantcomposed of a polyacid or a salt thereof; at least one compound selectedfrom erythritol, xylitol and pentaerythritol; and a dispersion medium.U.S. Publ. Pat. Appl. No. 20060223976 provides a conductive polymerexcellent in conductivity, heat resistance and moisture resistance, byincluding at least one organic sulfonate having an anion of an organicsulfonic acid, that is the same or different from the organic sulfonicacid incorporated in the conductive polymer as a dopant, and a cationother than transition metals.

Another approach is disclosed in U.S. Publ. Pat. Appl. No. 2010/0091432wherein the conductive polymer includes a conductive polymer, apolyanion that includes a hydrophilic group, where the polyanionfunctions as a dopant of the conductive polymer. Further, at least apart of the hydrophilic group of the polyanion is condensed with anepoxy group in a compound with one epoxy group.

Another problem with conductive polymer containing solid electrolyticcapacitor is the high leakage at high temperatures and after surfacemount conditions. It is theorized that one of the causes of this highleakage under these conditions is the lack of sufficient moisture in theICP coating. A lack of moisture in the ICP material during processes,such as surface mount, can cause leakage in solid electrolyticcapacitors.

U.S. Pat. No. 7,773,366 discloses a method for incorporating a waterretaining layer to improve leakage and other electrical characteristicsof a solid electrolytic capacitor. In this method, a water-retaininglayer having higher water absorption than that of the housing is placedbetween the conductive polymer layer and the housing. The waterabsorption of the housing is preferably 0.04% or less. Thereby, waterdissipated to the outside through the housing can be suppressed, and thereduction in the content of internal water can be prevented. As thewater-retaining layer, an epoxy resin can be used, and thewater-retaining layer can be formed by applying a liquid epoxy resin.

U.S. Publ. Pat. Appl. No. 2006/0240593 discloses a method for improvingleakage current by incorporating organic compounds having a boilingpoint of not lower than 150° C. and a melting point of no higher than150° C. While some of the above mentioned references claim moistureresistance improvement, other reference claim improved performance witha water retaining layer. However these approaches do not address theneed for a balance between moisture resistance/low moisture sorption andmoisture retention for solid electrolytic capacitor with improvedreliability. The above approaches also do not address issues relatedwith SMT reflow exposures.

Thus, there is a need for a process for forming solid electrolyticcapacitors with improved leakage and leakage stability. A particularneed is for capacitor parts to have stable leakage during surface mounttemperatures.

Thus, a need exists for the proper management of moisture which isrequired to produce solid electrolytic capacitors with excellentreliability. Here to fore it has not been recognized that a delicatebalance of moisture content and moisture retention properties arerequired to simultaneously avoid poor package integrity and high leakagecurrent after being subjected to SMT conditions.

SUMMARY

It is an object of the invention to provide an improved solidelectrolytic capacitor.

It is another object of the invention to provide an improved method ofpreparing a solid electrolytic capacitor cathode with good reliability.

It is another object of the invention to provide an improved method ofpreparing a solid electrolytic capacitor cathode with good leakagecurrent when exposed to high temperature conditions.

It is another object of the invention to provide an improved method ofpreparing solid electrolytic capacitor cathodes with good packageintegrity when exposed to SMT conditions.

These and other advantages, as will be realized, are provided acapacitor. The capacitor has an anode and a dielectric over the anode. Aconductive polymer layer is over the dielectric wherein the conductivepolymer layer has a moisture content of at least 16 wt % and a moistureloss of no more than 5 wt % upon heating from 125° C. to 175° C.

Yet another embodiment is provided in a capacitor with an anode and adielectric over the anode. A first conductive polymer layer is over thedielectric wherein the first conductive polymer layer comprises apolyanion and a first binder. A second conductive polymer layer is overthe first conductive polymer layer wherein the second conductive polymerlayer comprises a polyanion and a second binder and wherein the firstbinder is more hydrophilic than the second binder.

Yet another embodiment is provided in a capacitor with an anode and adielectric over the anode. A first conductive polymer layer is over thedielectric wherein the first conductive polymer layer has a firstmoisture content. A second conductive polymer layer is over the firstconductive polymer layer wherein the second conductive polymer layer hasa second moisture content. The first moisture content is at least 5 wt %higher than the second moisture content.

Yet another embodiment is provided in a capacitor with an anode and adielectric over the anode. A first polymer layer is over the dielectricand a second polymer is over the first polymer layer. The first polymerlayer and said second polymer layer have a moisture loss of less than 5wt % upon heating from 125° C. to 175° C.

Yet another embodiment is provided in a method for forming a capacitorcomprising:

providing an anode with a dielectric coating thereon;applying a first dispersion over the dielectric thereby forming a firstconductive polymer layer over the dielectric coating wherein the firstconductive polymer layer comprises a hydrophilic material; andapplying a second dispersion over the first conductive polymer layerthereby forming a second conductive polymer layer over the firstconductive polymer layer.

Yet another embodiment is provided in a capacitor with an anode and adielectric over the anode. A first conductive polymer layer is over thedielectric wherein the first conductive polymer layer has a moisturecontent of at least 20 wt %. A second conductive polymer layer is overthe first conductive polymer layer wherein the second conductive polymerlayer comprises at least one material selected from a hydrophobicmaterial and a moisture retaining compound.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a schematic cross-sectional view of an embodiment of theinvention.

FIG. 2 is a flow-chart schematic representation of an embodiment of theinvention.

FIG. 3 graphically illustrates the effect of moisture on leakage currentafter simulated SMT.

DESCRIPTION

The present invention is related to an improved capacitor and a methodfor making the improved capacitor. More particularly, provided herein isa method that allows the production of capacitors with improvedreliability.

The invention will be described with reference to the figures which forman integral non-limiting part of the specification.

An embodiment of the invention will be described with reference toFIG. 1. In FIG. 1, a capacitor, generally represented at 10, isillustrated schematically in cross-sectional view. The capacitorcomprises an anode, 12, which will be more fully described herein. Adielectric, 14, is formed on the anode which is preferable an oxide ofthe anode. A first conductive polymer layer, 16, is formed on thedielectric. The first conductive polymer layer may represent a singlelayer, such as applied by a single application, or multiple layers ofsimilar composition. A second conductive polymer layer, 18, is formed onthe first conductive polymer layer. The second conductive polymer layermay represent a single layer, such as applied by a single application,or multiple layers of similar composition. The first conductive polymerlayer and second conductive polymer layer will be described with moredetail herein. An adhesive layer, 20, provides a surface to which anexternal cathode termination, 22, may be attached such as by solder, 24.The adhesive layer typically comprises multiple sub-layers including atleast one carbon layer, which readily adheres to the second conductivepolymer layer, followed by at least one metal containing layer whichreadily adheres to the carbon layer and which can be soldered. An anodelead wire, 26, extends from the anode and is in electrical contact withan external anode termination, 28. The entire capacitor, except for thelower portion of the external cathode termination and external anodetermination, is preferable encased in a non-conductive resin, 30.

An embodiment of the invention will be illustrated with reference toFIG. 2. In FIG. 2 a flow chart representation of the process for forminga capacitor is provided. An anode is formed at 200. The anode can beformed by pressing a powder into a monolith or by forming a sheet ofconductive material. If a monolith is used it is preferable to attach ananode lead wire to the anode. A dielectric is formed on the anode at202. The dielectric is preferably an oxide of the anode formanufacturing convenience but is not limited thereto. At least one layerof a first conductive polymer layer is formed at 204. The polymer layercan be applied by in-situ polymerization or by dipping in a dispersionor slurry of conductive polymer. Dipping in a slurry of conductivepolymer is most preferred herein due to the intended incorporation ofhydrophilic enhancing materials. At least one layer of a secondconductive polymer layer is formed at 206. The second conductive polymerlayer can be applied by in-situ polymerization or by dipping in adispersion or slurry of conductive polymer. Dipping in a slurry ofconductive polymer is most preferred herein due to the intendedincorporation of hydrophilic enhancing materials. Adhesive layers areapplied at 208. The adhesive layers provide a surface upon which anexternal lead can be attached. Though not limited thereto at least onecarbon layer followed by at least one metal containing layer istypically sufficient to demonstrate the instant invention. Externalterminations are applied at 210 if desired. External terminations are inelectrical contact with the cathode in one instance and the anode inanother and form the electrical contact between the conductors of thecapacitor and the circuit trace in an electronic device. The capacitoris finished at 212 which may include resin encapsulating, testing,packing and the like.

In direct contradiction to the expectations in the art, the presentinvention is specific to methods to retain the moisture content of theconductive polymer layer through the SMT process. The conundrum facingthose of skill in the art has always been the choice of having arelatively high moisture content, which was advantageous for leakagecurrent but which caused severe problems in an SMT process, or having alow moisture, which was advantageous for use in an SMT process but thecapacitors suffer from a high leakage current. The present inventionsolves the problem of the art by formulating conductive polymer layerswhich have a desired level of moisture and which is able to retain thatmoisture within the layer during SMT applications.

The first conductive polymer layer and second polymer layer work inconcert to retain moisture within a predefined level.

In one embodiment, the first conductive polymer layer has a moisturecontent which is higher than the moisture content of the secondconductive polymer and the second conductive polymer layer inhibitsmoisture from escaping there through. This is accomplished bydifferential hardening, differential surface tension of the layers, ofdifferential hydrophilicity/hydrophobicity of the layers. The firstconductive polymer layer preferable has a moisture content which is atleast 110% by weight of the moisture content of the second conductivepolymer layer. More preferably, the first conductive polymer layer has amoisture content which is at least 200% by weight of the moisturecontent of the second conductive polymer layer.

In one embodiment the first conductive polymer layer and secondconductive polymer layer have a moisture content of at least 10 to 30 wt% and a moisture loss of no more than 5% upon heating from 125° C. to175° C.

In one embodiment the first conductive polymer layer loses no more than5 wt % moisture upon heating from 125° C. to 175° C. It is particularlypreferred that the first conductive polymer layer has moisture retentiondefined by a moisture loss of no more than 3% upon heating from 125° C.to 175° C. more preferably no more than 2 wt % and even more preferablyno more than 1 wt %. By controlling the relative moisture retention ofthe layers the moisture which is in the first conductive polymer layer,closest to the dielectric, cannot release the moisture due to the secondconductive polymer layer effectively forming a barrier layer.

To accomplish the differential moisture content, and moisture retentionand of the first conductive layer and second conductive layer it ishighly preferable that the first conductive polymer layer be relativelyhydrophilic and the second conductive polymer layer be relativelyhydrophobic. The desire of the first conductive polymer layer to containand retain moisture is therefore enhanced by the desire of the secondconductive polymer layer to exclude moisture. With a carefully balancedcombination of layers the moisture content can be achieved which allowsfor low leakage current while insuring that the moisture is retainedthrough SMT operation. Moisture retention is enhanced by incorporatingmaterials which absorb and retain water into the conductive polymerlayer. Particularly suitable materials include hydrogels, molecularsieves, and molecular containers.

The term molecular sieve and molecular containers refers to a particularproperty of these materials which is the ability to selectively sortmolecules based primarily on a size exclusion process. A molecular sieveis a material containing tiny pores of a precise and uniform size thatis used as an adsorbent for gases and liquids. Molecules small enough topass through the pores are adsorbed while larger molecules are not. Itis different from a common filter in that it operates on a molecularlevel and traps the adsorbed substance. For instance, a water moleculemay be small enough to pass through the pores while larger molecules arenot, so water is forced into the pores which act as a trap for thepenetrating water molecules which are retained within the pores. Becauseof this, they often function as a desiccant. A molecular sieve canadsorb water up to 22% of its own weight.

The principle of adsorption to molecular sieve particles is somewhatsimilar to that of size exclusion chromatography, except that without achanging solution composition, the adsorbed product remains trappedbecause in the absence of other molecules able to penetrate the pore andfill the space, a vacuum would be created by desorption. This is due toa very regular pore structure of molecular dimensions. An example ofmolecular sieve is zeolite. Zeolites are the aluminosilicate members ofthe family of microporous solids known as “molecular sieves.” Themaximum size of the molecular or ionic species that can enter the poresof a zeolite is controlled by the dimensions of the channels. These areconventionally defined by the ring size of the aperture, where, forexample, the term “8-ring” refers to a closed loop that is built from 8tetrahedrally coordinated silicon (or aluminum) atoms and 8 oxygenatoms. These rings are not always perfectly symmetrical due to a varietyof effects, including strain induced by the bonding between units thatare needed to produce the overall structure, or coordination of some ofthe oxygen atoms of the rings to cations within the structure.Therefore, the pores in many zeolites are not cylindrical.

In addition to molecular sieves, a further enhancement of moistureretaining property can be obtained by incorporating certain inorganicparticles such as natural or synthetic clay and their derivates,cyclodextrins and their derivatives. Any inorganic or organic particleswith certain pore size, preferably 1-30 Angstroms, and polarity whichassists in retaining moisture can be used for this application. Anyorganic or inorganic particles with certain pore size and hydrogenbonding can be used for retaining moisture.

Hydrogels are a class of polymer materials than can absorb large amountsof water without dissolving. Solubility is prevented by physical orchemical crosslinks in the hydrophilic polymer. Hydrogels are defined astwo- or multicomponent systems consisting of a three-dimensional networkof polymer chains and water that fills the space between macromolecules.Depending on the properties of the polymers used, as well as on thenature and density of the network joints, such structures in anequilibrium can contain various amounts of water; typically in theswollen state the mass fraction of water in a hydrogel is much higherthan the mass fraction of polymer. Two general classes of hydrogels canbe defined. Physical gels, or pseudogels, have chains which areconnected by electrostatic forces, hydrogen bonds, hydrophobicinteractions or chain entanglements. Physical gels are non-permanent andusually they can be converted to polymer solutions by heating. Chemicalhydrogels have covalent bonds linking the chains.

A hydrogel is a network of polymer chains that are hydrophilic,sometimes found as a colloidal gel, in which water is the dispersionmedium. Hydrogels are highly absorbent natural or synthetic polymerswhich can contain over 99% by weight water. Hydrogel material compriseshydrophilic crosslinked network polymers which can form a hydrogen bondwith moisture. Hydrogel materials further comprise particles of hydrogelmaterials.

Examples of crosslinked hydrogel materials suitable for demonstration ofthe invention are materials comprising crosslinked polymers andparticles formed from vinyl pyrrolidone polymers and copolymers,chitosan polymers and blends, vinyl alcohol polymers and copolymers,vinyl acetate polymers and copolymers, hydroxyl functional cellulosicpolymers, acrylic acid polymers and their copolymers, acrylamidepolymers and their copolymers, functionalized nanoparticles of varioushydrophilic polymers which can form crosslink with other hydrophilicbinders. Conductive polymer based hydrogel can be formed by a minimalnumber of modification in the binder system of the conductive polymerdispersion. Methods include forming a crosslinked networked formed froma self crosslinked hydrophilic binder, crosslinks formed from ICPdispersion components and a very hydrophilic binder, hybrid crosslinksformed from two or more hydrophilic binders, semi-interpenetratingnetworks formed from two or more polymer networks interpenetrating eachother, ionically crosslinked hydrogels formed from crosslinking betweenone or more hydrophilic binders which can ionically crosslink with eachother or with ICP dispersion components.

Hydrogels must be able to hold, in equilibrium, certain amounts ofwater. This implies that the polymers used in these materials must haveat least moderate hydrophilic character. In practice, to achieve highdegrees of swelling, it is common to use synthetic polymers that arewater-soluble when in non-crosslinked form. Typical simple materialsapplied for general-purpose hydrogels are poly(ethylene oxide),poly(vinyl alcohol), polyvinylpyrrolidone and poly(hydroxyethylmethacrylate). Examples of hydrogel materials are polyurethane,poly(ethyleneglycol), poly(propylene glycol), poly(vinylpyrrolidone),hydroxyl ethyl cellulose, Xanthan, methyl cellulose, starch,poly(vinylpyrrolidone), poly(acrylic acid), carboxymethyl cellulose,hydroxypropyl methyl cellulose, polyvinyl alcohol, acrylic acid,methacrylic acid, chitosan, αβ-glycerophosphate, Hydrophilic polyesters,polyphosphazenes, polypeptides, chitosan, poly (vinyl methyl ether) andpoly(N-isopropyl acrylamide). A particularly suitable combination fordemonstrating the invention is polyethylene oxide, polyvinyl alcohol andhydroxyl ethyl cellulose.

A mixture of hydrogel materials and zeolite particles can be combined toobtain a synergistic moisture retaining property.

For retaining moisture at higher temperatures, such as above 200° C.,the zeolite or hydrogel can be encapsulated in a high temperaturepolymer or binder. The encapsulating polymer or binder is chosen suchthat it prevents moisture from releasing below certain high temperaturesand facilitates gradual release of moisture above this high temperature.This temperature is determined by the glass transition or softeningtemperature of the binder. The glass transition temperature of theencapsulating binder is preferably above 100° C., more preferably above150° C. and most preferably above 200° C. Above the glass transition orsoftening temperature, moisture permeability through the binder filmgradually increases. This controlled release prevents rapid release ofmoisture, but ensures that sufficient moisture is retained to providethe desired property of the device. High performance high Tg polymerssuch as polyimide and copolymers, polyetherether ketone (PEEK) andcopolymers, polyphenyl sulfone and copolymers, polysulfone andcopolyemrs, polypthalamine (PPA) and copolymers, polyamideimide (PAI)and copolymers, liquid crystal polymer (LCP), novolak based epoxyresins, cresol novolak epoxy resins etc. can be used. In addition highbarrier polymer such as polyvinylidine chloride can also be used asencapsulant.

The first conductive polymer layer is rendered relatively hydrophilic byincorporation of hydrophilic materials into the polymer layer. Thehydrophilic material is preferably added at a level of 5 wt % to no morethan 70 wt %. Below about 5 wt % the moisture retention properties areinsufficient to realize the advantages. Above about 70 wt % the layerconductivity decreases which is detrimental. The first conductivepolymer layer preferably has a hydrophilicity enhancing materialselected from hydrophilic polymers and hydrogels. The polymers may becross-linked and a cross-linked polymer is preferred in some instances.Particularly preferred hydrophilicity enhancing materials includepoly(ethylene oxide), poly(vinyl alcohol), poly(hydroxyethylmethacrylate), polyethyleneoxide-polyvinyl alcohol-cellulose,polyurethane, poly(ethyleneglycol), poly(propylene glycol),poly(vinylpyrrolidone), Xanthan, methyl cellulose, starch,poly(vinylpyrrolidone), poly(acrylic acid), carboxymethyl cellulose,hydroxypropyl methyl cellulose, polyvinyl alcohol, acrylic acid,methacrylic acid, chitosan, αβ-glycerophosphate, polyphosphazenes,polypeptides, poly (vinyl methyl ether) or poly(N-isopropyl acrylamide).One of the desirable requirements for these materials is good filmforming properties and the ability to form flexible films which arecrack resistant on mechanical or thermal stress. Some polyanions usedfor conductive polymers are hydrophilic but they tend to be brittle onexposure to stress and cannot be used reliably to enhancehydrophilicity.

The second conductive polymer layer is rendered relatively hydrophobicby incorporation of hydrophobic materials into the polymer layer. Thehydrophobic material is preferably added at a level of 5 wt % to no morethan 70 wt %. Below about 5 wt % the ability to function as a blockinglayer to moisture migration from the first conductive polymer layer isinsufficient to realize the advantages. Above about 70 wt % the layerconductivity decreases which is detrimental. The second conductivepolymer layer preferably has a hydrophobicity enhancing materialselected from thermosetting materials, fluoro-polymers and theircopolymers, silicone polymers and their copolymers, silicone epoxy,silicone polyester silicone modified polyimides, fluorinated polyimdies,crosslinkable acrylics, crosslinkable polyester, crosslinkable polyvinylbutyral, crosslinkable epoxies, hyperbranched hydrophobic polymers,hydrophobic silanes, or hydrophobically modified organic and inorganicparticles.

The second conductive polymer layer preferably has hydrophobicproperties and low permeability to moisture and can be used to preventmoisture from releasing from the first moisture retaining layer.Moisture permeability of the second conductive polymer layer can bedecreased by forming dense crosslinked networks formed from crosslinkingbetween binders, polyanions, and rheological additives. Interpenetratingnetworks of these materials where two or more polymeric chainsinterpenetrate each other can also decrease moisture permeability.Incorporation of nanoparticles in the conductive polymer dispersion alsocan decrease the permeability of moisture through these films.

Hydrophobicity of the conductive polymer dispersion can be modified byincorporating silicone and various materials to obtain a criticalsurface tension. Several methods are developed to improvehydrophobicity. One of the methods is the use of hydrophobicinterpenetrating networks in the conductive polymer dispersions.Silicone polymers can be made water soluble by incorporating ethyleneoxide functionality. When these silicone-ethylene oxide copolymers withhydroxyl groups react with a co-binder, the hydrophobic silicone groupblooms outward towards the interface thus imparting hydrophobicity.Perfluoro- and octyl-functionalized silane also behave similarly whenused as an additive. The silanol groups react with hydroxyl groups inthe system and the hydrophobic perflouro- or octyl-groups bloom outwardtoward the interface giving hydrophobicity. An interpenetrating networksystem where some of the available hydrophilic functional groups in theconductive polymer dispersion such as polyanion, rheological additives,and binder, are crosslinked with hydroxyl functional hydrophobicsilicone-ethylene oxide copolymers. These multiple crosslinks throughmultiple polymeric chains create an interpenetrating network with lowpermeability to water. Additionally, the composition may also contain amixture of perfluoro- and octyl-functionalized silane and multifuctionalsilane, or dipodal silane, where the critical surface tension of thesilane is below 35 dynes/cm.

In one embodiment the conductive polymer dispersion comprises aconductive polymer and a mixture of polyanion where one of thepolyanions contains at least one sulfonic acid and one carboxilic acid,a polymeric binder which contains hydroxy functional groups, where oneof the binders is preferably a hydroxyl functional silicone-ethyleneoxide copolymer, a rheology additive containing carboxylic and hydroxyfunctional groups, a mixture of silane and dipodal silane couplingagents where one of the silane has a critical surface tension below 35dynes/cm, a crosslinking agent containing at least one carboxylic groupwhere the crosslinking reaction results in an ester formation betweenpolyanion and binder and rheology additive and the crosslinker, wherethe presence of multiple crosslinking networks through multiplepolymeric chains creates hydrophobic interpenetrating networks.

Both silicone copolymer and hydrophobic silane incorporation intoconductive polymer dispersion result in an conductive polymer dispersioncoating with a critical surface tension suitable for the secondconductive polymer layer.

The anode is a conductor and more preferably a valve metal or conductiveoxide of a valve metal with tantalum, aluminum, niobium and niobiumoxide being mentioned as particularly preferred. An advantage of thehigh surface area is that a very high capacitance density can beachieved.

Conjugated polymers are particularly suitable for use as theelectrically conductive solid cathode with polyaniline, polypyrroles andpolythiophenes being most preferred. A particularly preferred polymerfor use as a cathode is polythiophene. The polymer precursors arepolymerized to form the conductive layer which functions as the cathodeof the capacitor. The polymer precursors are preferably polymerized byeither electrochemical or chemical polymerization techniques withoxidative chemical polymerization being most preferred. In oneembodiment, the conductive layer is formed by dipping the anodizedsubstrate first in a solution of an oxidizing agent such as, but notnecessarily limited to iron (III) p-toluenesulfonate. After a dryingstep, the anode bodies are then immersed in a solution comprisingmonomer and oligomer of the conductive polymer and solvents.

It is preferred to include a dopant in the polymer as known in the art.The dopant can be coated separately or included in the polymer slurry ormonomer solution. A particularly preferred dopant is the sodium salt ofpolystyrenesulfonate (PSS).

The conducting polymer is preferably an intrinsically conducting polymercomprising repeating units of a monomer of Formula I and optionally anoligomer Formula II:

R¹ and R² of Formula I and R⁴-R⁹ of Formula II are chosen to prohibitpolymerization at the α-site of the ring. It is most preferred that onlyα-site polymerization be allowed to proceed. Therefore, it is preferredthat R¹ and R² are not hydrogen. More preferably R¹, R², R⁴, R⁵, R⁶, R⁷,R⁸ and R⁹ are α-directors. Therefore, ether linkages are preferable overalkyl linkages. It is most preferred that the groups are small to avoidsteric interferences. For these reasons R¹ and R², R⁴ and R⁵, R⁶ and R⁷or R⁸ and R⁹ each taken together as —O—(CH₂)₂—O— is most preferred.

In Formula II n is an integer selected from 0-3.

In Formulas I and II, X and Y independently are S, Se or N. Mostpreferably X and Y are S.

R¹, R², R⁴, R⁵, R⁶, R⁷, R⁸ and R⁹ independently represent linear orbranched C₁-C₁₆ alkyl or C₁-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl,phenyl or benzyl which are unsubstituted or substituted by C₁-C₆ alkyl,C₁-C₆ alkoxy, halogen or OR³; or R¹ and R², R⁴ and R⁵, R⁶ and R⁷ or R⁸and R⁹, taken together, are linear C₁-C₆ alkylene which is unsubstitutedor substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen, C₃-C₈ cycloalkyl,phenyl, benzyl, C₁-C₄ alkylphenyl, C₁-C₄ alkoxyphenyl, halophenyl, C₁-C₄alkylbenzyl, C₁-C₄alkoxybenzyl or halobenzyl, 5-, 6-, or 7-memberedheterocyclic structure containing two oxygen elements. R³ preferablyrepresents hydrogen, linear or branched C₁-C₁₆ alkyl or C₁-C₁₈alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl.

More preferably R¹, R², R⁴, R⁵, R⁶, R⁷, R⁸ and R⁹, independently of oneanother, represent —CH₃, —CH₂CH₃; —OCH₃; —OCH₂CH₃ or most preferably R¹and R², R⁴ and R⁵, R⁶ and R⁷ or R⁸ and R⁹ are taken together torepresent —OCH₂CH₂O— wherein the hydrogen can be replaced with asolubilizing group, a halide or an alkyl.

The polymer layer can be formed by in-situ polymerization on the surfaceor by applying a slurry of polymer onto the dielectric coated such as bydipping or spraying. These techniques are described in the literatureand are widely understood and will not be further explained herein.Achieving adequate coverage of the edges and corners is difficult as setforth above. The choice of method for forming the polymer layer isselected based, in part, on the location of the layer being formed. Itis widely known that a conductive polymer layer is typically formed bymultiple applications of thinner layers each of which may be formed byeither in-situ polymerization or by slurry dipping. Materials whichenhance corner and edge coverage can be employed and may be beneficialto the overall reliability of the capacitor but are not necessary fordemonstration of the instant invention.

After the desired numbers of polymer layers are formed, or the desiredthickness is achieved, it is preferable to apply layers which facilitateadhesion to a lead frame. Conductive carbon layers followed byconductive metal layers, particularly noble metal or semi-noble metallayers are formed on the conductive polymer. Conductive carbon layerscan also be rendered hydrophobic to help retain moisture in theconductive polymer layer. Carbon can be made hydrophobic by use ofhydrophobicity enhancing materials described above. Carbon layer canfurther comprise moisture retaining compounds to further improvemoisture retention of the cathode layer. The capacitor is finished afterattaching to external leads, encapsulation, testing, packaging and thelike.

The moisture content determination was done using the Karl Fischermethod which is a coulometric titration method using Karl Fischerreagent, which reacts quantitatively and selectively with water, tomeasure moisture content. Karl Fischer reagent consists of iodine,sulfur dioxide, a base and a solvent, such as alcohol. In this test, themoisture containing sample can be mixed with Karl Fischer reagentdirectly in the titration vessel. Alternatively, the sample can beheated to a higher temperature and the moisture vapor then reacts withthe Karl Fischer reagent in the titration vessel. The amount of water isdirectly proportional to the iodine consumed in the reaction. In thecoulometric titration method, iodine is produced through an electrolyticoxidation and can be measured by the quantity of the electricity used.The water content is therefore determined from the coulombs required inthe titration. 5 g of the dispersion sample was dried in a 4 inch Al panat 150° C. in a convection oven for 30 minutes, followed by conditioningat RH71%, 74° F. for 48+ hours. The samples were sealed until testing.The sample was placed in a Karl Fischer sample holder and the titrationwas done at 250° C. In a comparative study the moisture content by KarlFischer the method was determined to be 17-18% for the control sampleusing Clevious KV2 which is a widely used conductive polymer forcapacitors.

Moisture content and moisture loss are determined on a film wherein thefilm has identical composition to that in the capacitor layer beinganalyzed. Therefore, the percent moisture, percent moisture loss, etc.can be easily determined experimentally with the results considered tobe representative of the same results as would be obtained in acapacitor.

The moisture content and moisture loss at different temperatures aremeasured on a film formed from the conductive polymer dispersion usingthe following method. 5 g of the dispersion sample was dried in a 4 inchAl pan at 150° C. in a convection oven for 30 minutes, followed byconditioning at RH71%, 74° F. for 48+ hours. The conditioned film wasplaced in a thermogravimetric analyzer (TGA) and heated at scan rate of40° C./min to reach the desired temperature (T). At the temperature (T),the sample was kept for 30 min and the weight loss was measured. The dryweight (Wdry) is determined using the TGA. The moisture content wasdetermined for other states by repeating the steps except that adifferent drying temperature (T) was used such that 100° C.<T<200° C.The weight (WT) of the film after 30 min at temperature (T) was plottedand moisture content at temperatures of interest was determined byinterpolation. The weight content and moisture retention was determinedby the equations:

Wdry=weight of film after isothermal TGA at 200° C.

WT=wt of film at specified temperature T using isothermal TGA

Moisture content T=100*(WT−Wdry)/WT

Moisture retention=Moisture content at T2−Moisture content T1 whereinT1>T2

Moisture content determined by the TGA method was determined to be 17%for the control, using Clevios KV2, based on the assumption that allmoisture is lost at 200° C. after 15 min. Moisture loss was measuredusing the following method.

Using isothermal TGA for the sample was run at various temperatures anda regression equations for each sample is generated. Using theregression equations, weight loss at four different temperatures wasdetermined for control and inventive samples, and reported in Table 1.

TABLE 1 Wt loss after 30 min at Wt loss after 30 min temperature attemperature TGA temperature Control (clevios KV2) Inventive 100 −9.1017−13.946 125 −12.1342 −14.966 150 −15.1667 −15.986 175 −18.1992 −17.006

The moisture loss at 175° C. relative to 125° C. for the control wasdetermined as −18.1−(−12.1)=−6 and for the inventive the moisture lossunder the same conditions was calculated as −17−(−14.9)=−2.1% whichshows a significant improvement for the inventive example.

Comparative Example 1

Tantalum anodes were anodized to form a dielectric on the tantalumanode. The anode thus formed was dipped into a solution of iron (Ill)toluenesulfonate oxidant for 1 minute and sequentially dipped intoethyldioxythiophene monomer for 1 minute. The anodes were washed toremove excess monomer and by-products of the reactions after thecompletion of 60 minutes polymerization, which formed a thin layer ofconductive polymer (PEDOT) on the dielectric of the anodes. This processwas repeated 6 times. A commercial conductive polymer dispersion,available as Clevios KV2, was applied 2-4 times to form a thick externalpolymer layer. A conventional graphite coating was applied followed by asilver layer.

Inventive Example 1

Parts were prepared in the same manner as in Comparative Example 1except the conductive polymer dispersion was prepared with a hydrophilicpolymer, polyethylene oxide (designated as HL). A conventional graphitecoating was applied followed by a silver layer. Parts were assembled andESR before and after surface mount was measured. Moisture content versusleakage current was determined and is reported in Table 2.

Inventive Example 2

Parts were prepared in the same manner as in Comparative Example 1except the conductive polymer dispersion was prepared with an oligomerwith hydroxyl and carboxylic groups available for crosslinking,designated as HP. A conventional graphite coating was applied followedby a silver layer. Parts were assembled and ESR before and after surfacemount was measured. Moisture content versus leakage current wasdetermined and is reported in Table 2.

TABLE 2 Moisture content % Leakage (KF method) (microamp) ComparativeExample 18 191 Inventive Example 1 (HL) 24 122 Inventive Example 2 (HP)13 346

Inventive Example 3

Parts were prepared in the same manner as in Comparative Example 1except the conductive polymer dispersion was prepared with a mixture ofpolymers comprising polyethylene oxide, polyvinyl alcohol, andhydroxyethyl cellulose, designated as HLx. A conventional graphitecoating was applied followed by a silver layer. Parts were assembled andESR before and after surface mount was measured. The moisture contentwas determined by the Karl Fischer method to be 24 wt % versus 18 wt %for Comparative Example 2. The effect of moisture content on leakageafter post stimulated SMT at 260° C. with two cycles is illustrated inFIG. 3.

Inventive Example 4

Parts were prepared in the same manner as in Comparative Example 1except that a dual layer of conductive polymer dispersion was used. Thefirst conductive polymer layer was prepared from a hydrophilic polymer,HL, and second conductive polymer layer was prepared from a hydrophobicpolymer, HP crosslinkable polyester. Five groups of capacitors wereprepared with various layer structures, each designated with a numberindicating number of layers from each category. HL3/HP2 indicates 3layers of HL conductive polymer dispersion and 2 layers of hydrophobicpolymer. A conventional graphite coating was applied followed by asilver layer. The effect of the combined layered structures on leakageand equivalent series resistance (ESR) is provided in Table 3.

TABLE 3 Conductive dispersion Leakage layer structure (microamp) ESR(mohms) HL5 25.18 26.85 HL3/HP2 37.10 26.99 HL2/HP3 82.17 29.48 HL1/HP4219.39 28.68 Control 5 442.86 32.33

The results presented in Table 3 demonstrate the advantages offered bythe invention.

The invention has been described with reference to the preferredembodiments without limit thereto. One of skill in the art would realizeadditional embodiments and alterations which are not specifically statedbut which are within the scope of the invention as set forth in theclaims appended hereto.

1-72. (canceled)
 73. A method for forming a capacitor comprising:providing an anode with a dielectric coating thereon; applying a firstdispersion over said dielectric thereby forming a first conductivepolymer layer over said dielectric coating wherein said first conductivepolymer layer comprises a dopant and a hydrophilic material; andapplying a second dispersion over said first conductive polymer layerthereby forming a second conductive polymer layer over said firstconductive polymer layer.
 74. The method for forming a capacitor ofclaim 73 wherein said second conductive polymer layer comprises ahydrophobic material.
 75. The method for forming a capacitor of claim 74wherein said hydrophobic material is selected from the group consistingof thermosetting materials, fluoro-polymers and their copolymers,silicone polymers and their copolymers, silicone polyester,crosslinkable polyester and crosslinkable epoxies.
 76. The method forforming a capacitor of claim 75 wherein said hydrophobic material ispolytetrafluoroethylene.
 77. The method for forming a capacitor of claim73 wherein said first conductive polymer layer has a moisture content ofat least 21 wt %.
 78. The method for forming a capacitor of claim 73wherein said second conductive polymer layer has a moisture content ofno more than 15 wt % at 125° C.
 79. The method for forming a capacitorof claim 73 wherein said hydrophilic material selected from the groupconsisting of poly(ethylene oxide), poly(vinyl alcohol), hydroxyl ethylcellulose, polyvinylpyrrolidone, poly(hydroxyethyl methacrylate),polyurethane, poly(ethyleneglycol), poly(propylene glycol),poly(vinylpyrrolidone), Xanthan, methyl cellulose, starch,poly(vinylpyrrolidone), poly(acrylic acid), carboxymethyl cellulose,hydroxypropyl methyl cellulose, polyvinyl alcohol, acrylic acid,methacrylic acid, chitosan, aJ3-glycerophosphate, hydrophilic polyester,polyphosphazene, polypeptide, chitosan, poly (vinyl methyl ether) andpoly(N-isopropyl acrylamide).
 80. The method for forming a capacitor ofclaim 79 wherein said first conductive polymer layer comprises at leastone hydrophilic material selected from the group consisting ofpoly(ethylene oxide), poly(vinyl alcohol) and hydroxyl ethyl cellulose.81. The method for forming a capacitor of claim 79 wherein saidhydrophilic material is crosslinked.
 82. The method for forming acapacitor of claim 81 wherein said hydrophilic material is crosslinkedwith a crosslinking agent.
 83. The method for forming a capacitor ofclaim 82 wherein said crosslinking agent comprises functional groupsselected from hydroxyl and carboxyl.
 84. The method for forming acapacitor of claim 81 wherein said hydrophilic material is crosslinkedwith a conductive polymer of said first conductive polymer layer. 85.The method for forming a capacitor of claim 73 wherein at least one ofsaid first conductive polymer or said second conductive polymercomprises a moisture retaining component.
 86. The method for forming acapacitor of claim 85 wherein said moisture retaining component is amaterial selected from the group consisting of a hydrogel, a molecularsieve, and a molecular container.
 87. The method for forming a capacitorof claim 85 wherein at least said moisture retaining component isencapsulated in a polymeric encapsulant.
 88. The method for forming acapacitor of claim 87 wherein said polymeric encapsulant has a Tg of atleast 100° C.
 89. The method for forming a capacitor of claim 88 whereinsaid polymeric encapsulant has a Tg of at least 150° C.
 90. The methodfor forming a capacitor of claim 73 wherein at least one of said firstconductive polymer layer or said second conductive polymer layer has amoisture loss of no more than 5 wt % upon heating from 125° C. to 175°C.
 91. The method for forming a capacitor of claim 90 wherein saidmoisture loss is no more than 3 wt %.
 92. The method for forming acapacitor of claim 90 wherein said moisture loss is no more than 1 wt %.93. The method for forming a capacitor of claim 73 wherein said firstconductive polymer layer has a moisture content which is at least 110 wt% of a moisture content of said second conductive polymer layer.
 94. Themethod for forming a capacitor of claim 93 wherein said first conductivepolymer layer has a moisture content which is at least 150 wt % of themoisture content of said second conductive polymer layer.
 95. The methodfor forming a capacitor of claim 94 wherein said first conductivepolymer layer has a moisture content which is at least 200 wt % of themoisture content of said second conductive polymer layer. 96-117.(canceled)